Light emitting diodes are one type of semiconductor devices within the category known as photonic or optoelectronic devices. Other photonic devices include photodetectors (i.e., devices that detect optical signals) and photovoltaic devices (i.e., devices that convert optical radiation into electrical energy).
Light emitting diodes have gained wide acceptance and usage for reasons similar to those behind the wide ranging acceptance and uses of other semiconductor devices: their small size, relatively low cost, low power requirements, and many other factors that have driven growth in all segments of the electronics industry in the past few decades.
The colors that can be produced by a light emitting diode, however, are fundamentally related to the semiconductor material from which the LED is formed. In turn, the fundamental property of the material that defines the available colors is the material's bandgap. As is well known to those familiar with electronic transitions, the bandgap is the energy difference between the valence band and the conduction band of a semiconductor material. Thus, the transitions between bands give off light of a certain energy, and the material's bandgap represents the maximum light energy that the material can produce. In turn, the energy of such light--alternatively expressed as the energy of the photon produced by the bandgap transition--is expressed as its wavelength and frequency. In the visible portion of the electromagnetic spectrum, different wavelengths (which is inversely proportional to the energy emitted) and frequencies (which is directly proportional to the energy emitted) appear as different colors to the human eye.
Obtaining accurate primary colors such as red, blue, and green from LEDs is a desirable goal as various full color devices (e.g., displays, lasers, photocopiers, detectors, etc.) cannot operate most effectively and efficiently if the primary colors are unavailable. Instead, various light filtering and blending techniques must be used.
The green portion of the visible spectrum; i.e., that portion that defines light that will appear to human eyesight as the color green, is between about 492 and 577 nanometers (nm). See; Sze, Physics of Semiconductor Devices, 2d Ed. (1981) p. 683. To date, most "green" LEDs are formed of gallium phosphide (GaP) doped with nitrogen (N). The standard transition in gallium phosphide produces a 565 nm photon which is well towards the yellow portion of the spectrum and thus the emitted light has more of a yellow-green appearance than a true green appearance. Some gallium phosphide light emitting diodes have produced emissions at about 555 nm, but none have demonstrated emission at shorter wavelengths (higher frequencies) that would produce a more true green color.
Silicon carbide is a ideal candidate material for semiconductor devices, and particularly for LEDs requiring relatively large bandgaps in order to produce colors in the higher energy portion of the visible spectrum; i.e. green, blue and violet. For example, the use of silicon carbide to successfully produce blue light emitting diodes (i.e., higher energy, higher frequency, and shorter wavelength than green light) is set forth in U.S. Pat. Nos. 4,918,497 and 5,027,168 both to Edmond, and both assigned to the assignee of the present invention. Because silicon carbide has an appropriately wide bandgap, 3.0 electron volts (eV) at room temperature, it can theoretically provide transitions that will produce photons of any color in the visible spectrum, and indeed some into the ultraviolet (UV) portions of the electromagnetic spectrum.
A specific color, however, requires a transition of a specific energy. Thus, a true green photon with a wavelength of about 530 nm, must be produced by a transition of about 2.3 eV. This transition is within the bandgap of silicon carbide, but does not represent silicon carbide's full bandgap. Thus, some mechanism must be provided for 530 nm events to occur within the larger bandgap of silicon carbide. Page 684 of Sze, supra, gives a brief discussion of such mechanisms, which are well understood by those of ordinary skill in this art.
A number of attempts at the true green 530 nm LED have been carried out to date. In 1986, Demitriev et al., Sov Tech. Phys. Lett. 12(5) May 1986, p. 221, reported on a three color display formed in silicon carbide in which epitaxial layers were produced by liquid phase epitaxy and ion implantation. Stinson, U.S. Pat. No. 4,992,704 describes a variable color light emitting diode, but offers no explanation or suggestion as to how the green diode he incorporates could be produced or its structure.
Suzuki et al., U.S. Pat. No. 5,063,421 describes a silicon carbide LED that emits in potential color ranges from green to purple in which a tetravalent transition element (titanium, zirconium, or hafnium) acts as the luminescent center. Vodakov et al., Sov. Phys. Semicond. 26(1) January 1992, pp. 59-61, describe "pure green" silicon carbide LEDs in the 530 nm wavelength, produced by "sublimation epitaxy."
In a later effort, Vodakov et al. Sov Phys. Semicond. 26(11) November 1992, pp. 1041-1043, report silicon carbide diodes that produce in the 510-530 nm region grown by vapor phase epitaxy and then bombarded with fast electrons.
Niina et al., U.S. Pat. No. 5,187,547 describes a specific silicon carbide LED structure that it provides a 482 nm peak wavelength. Those familiar with such devices will, however, recognize this as being blue emitting rather than green.
Suzuki et al., U.S. Pat. No. 5,243,204 discloses several green emitting LEDs in silicon carbide, one of which Suzuki describes as a pair of epitaxial layers of 3C (beta) silicon carbide on a beta silicon carbide substrate and an emission at a peak wavelength of 544 nm. Bulk single crystals of beta silicon carbide have yet to make a wide appearance, however, and Suzuki's example may be predictive, rather than descriptive.
Kaise et al., U.S. Pat. No. 5,302,839 is an example of a gallium phosphide green light emitting diode.
Neudeck et al., IEEE Electron Device Letters, 14(3) March 1993, pp. 136-139, describe 3C silicon carbide junction diodes, but confirm that there is a general lack of 3C substrates suitable for mass production. Thus, the diodes described by Neudeck et al. are all grown on 6H (alpha) silicon carbide substrates.
Finally, Suvorov et al., High-Effective Ion-Implanted Green GH-SiC LEDs, describe green LEDs produced from 6H silicon carbide through ion implantation to produce the required p-n junctions. The described device had wavelengths of between 530-540 nm, but were considered to have unsatisfactory electrical characteristics and effectiveness. In particular, none of these devices showed outputs greater than about 15 or 20 microwatts at the standard measuring current of 20 milliamps.